Copolymers of olefin such as ethylene and propylene which is a nonpolar monomer and a vinyl monomer containing polar group have been widely known. Specifically, ethylene-vinyl alcohol copolymers (EVOH) are random copolymer comprising ethylene and vinyl alcohol and synthesized by saponifying ethylene-vinyl acetate copolymers obtained by radical copolymerization of ethylene and vinyl acetate. EVOH is used in a wide range of fields for purposes such as food packages by taking advantage of its excellent gas barrier property.
It is widely known that copolymers obtained by copolymerization of ethylene through radical polymerization generate short-chain branches and long-chain branches by back biting reaction. For example, in the case of EVOH, it has been reported that about 1 mol % of alkyl branches and about 0.1 to 0.2 mol % of acetoxyl branches are present in EVOH containing about 30 mol % of ethylene (Nihon Kagaku Gakkaisi, 11, 1698 (1977)). It is known that generally, presence of branches in the polymer chain incurs decrease in the degree of crystallization and changes in the properties of the polymer.
The polymerization of monomers containing allyl group is more difficult compared to that of vinyl monomers, and the polymer of allyl group-containing monomers has been almost unheard. The main reason for this is that the polymer propagation reaction proceeds very slowly due to the degenerative chain transfer reaction to monomer and hence only oligomers having low degree of polymerization have been obtained (Chem. Rev. 58, 808 (1958)).
JP-A-S58-49792 discloses a copolymer of ethylene and allyl acetate and a tripolymer of ethylene, allyl acetate and vinyl acetate as a hydrocarbon oil composition. The synthesizing method thereof is radical polymerization, and a low-molecular-weight substance having limiting viscosity of about 0.12 dl/g was obtained in Examples.
JP-A-2005-514083 discloses synthesis of ethylene-allyl alcohol copolymer aiming for higher hydrophobicity compared to EVOH as a coating material for medical instruments. The synthesis method is different from that of the present invention, which aims to directly obtain polymers by polymerization of allyl monomers, and the targeted polymer is obtained in JP-A-2005-514083 by reduction reaction after the radical copolymerization of ethylene and acrylic acid. However, the method had a problem that the reduction reaction of the polymer cost too much. Furthermore, since the polymer is synthesized by radical polymerization, the polymer skeleton is presumed to have a branched structure.
Copolymerization of polar group-containing monomer by coordination polymerization using a Ziegler-Natta catalyst and a metallocene catalyst is difficult to conduct under general conditions since the polar group becomes a catalyst poison, which is different from radical polymerization. U.S. Pat. No. 4,423,196 (Patent Document 1) discloses copolymers of propylene and allyl alcohol which are obtained by polymerization using TiCl3-type Ziegler-Natta catalyst. The polymerization reaction proceeds by using equimolar organic aluminum compound to allyl alcohol and by protecting the alcohol moiety with organic aluminum. Though the publication does not have description regarding the molecular weight distribution, the polymer contains 98% of isotactic fraction and is presumed to be a polymer having a wide-range of molecular weight distribution and composition distribution.
Polymerization of nonpolar vinyl monomer such as ethylene and propylene and polar monomer has also been attempted using single-site catalysts which have been developed recent years.
It has been conventionally known that the catalyst using metal complex of group 4 elements has high polymerization activity to monomers such as ethylene and propylene and there has also been a disclosure of copolymerization of polar group-containing monomers. In the copolymerization of ethylene and polar group-containing monomer using a metallocene catalyst of group 4 elements, it was necessary to use organic aluminum in at least an equimolar amount to allyl alcohol, which aluminum functions as a protecting group for the polar group-containing monomer against the catalyst. As a result, the reaction of chain transfer to organic aluminum dominantly terminated the propagation reaction, and only a saturated terminal bond was observed in a terminal structure of the polymer while a terminal double bond by β-hydrogen elimination was not. In this case, it leads to cost increases due to the use of organic aluminum in excess; the fact that the copolymerization of the polar group-containing monomers cannot be high by the factor that the concentration of polar group-containing monomers cannot be increased; and cost increases in recovering unreacted monomers after the polymerization reaction, which become problems in achieving practical use of the method.
JP-A-2003-252930 (Patent Document 2) and J. Am Chem. Soc., 124, 1176 (2002) (Non-patent Document 1) disclose an olefin polymer containing two polar groups at position of ω of the main chain of the olefin polymer using metallocene complex of group 4 elements having a specific structure; an olefin polymer containing a polar group at position of ω and at least one position of (ω-n) (n≧1); and a production method thereof. By the analysis of the terminal structure of the polymer, it has been confirmed that only a saturated bond exists at the molecular chain terminal while an unsaturated bond does not. The allyl alcohol content in the polyethylene main chain of the copolymer obtained by copolymerization of ethylene and allyl alcohol using a zirconocene catalyst having a specific structure, which copolymer described in Examples, is within the range of from 0.2 to 1.2 mol %. Also, organic aluminum is used in at least an equimolar amount to allyl alcohol.
JP-A-2006-265541 (Patent Document 3) describes a method for producing a polar olefin copolymer using a metal complex of groups 4 to 5 elements having a specific structure. Examples disclose copolymerization of ethylene and allyl chloride, ethylene and allyl acetate, and ethylene and allyl alcohol. In the copolymerization of ethylene and allyl chloride, the allyl content in the main chain of polyethylene is from 0.1 to 0.3 mol %; and organic aluminum is used in at least an equimolar amount to an allyl compound.
JP-A-2003-231710 (Patent Document 4) discloses a method for producing a copolymer of olefin and a polar vinyl monomer using a catalyst comprising a lamellar compound. Examples describe copolymerization of propylene and allyl alcohol, wherein the allyl alcohol content in the polymer is as small as 0.3% or less and organic aluminum is used.
It is generally known that the polar group-containing monomer can be copolymerized without using organic aluminum as a protective group in the catalyst system using late transition metal. Examples include copolymerization of ethylene and acrylic acid ester, acrylonitrile, vinyl acetate and the like (J. Am. Chem. Soc., 118, 267 (1996) (Non-patent Document 2); J. Am. Chem. Soc., 129, 8948 (2007) (Non-patent Document 3; JP-A-2007-046032 (Patent Document 5)). However, conventionally, not only that activity is low but that polymerization activity reduces during the long time period of polymerization in the catalyst system using late transition metal, and therefore the cost of catalyst using expensive late transition metal complex is quite high and the method has a problem to be industrially used.
On the other hand, in the case of allyl compound, the copolymerization reaction of an allyl compound and olefin, which is an objective of the present invention, has been almost unheard because the reaction could proceed in a different format other than the polymerization reaction at olefin moiety: i.e. an oxidative addition reaction of an allyl compound to late transition metal.